1. Field of the Invention
The present invention relates to a device for processing the phase information of an acoustic signal and a method thereof, and more particularly, to a device for processing the phase information of an acoustic signal, by which important phase components are discriminated in consideration of human auditory recognition characteristics, and a method thereof.
2. Description of the Related Art
Research into auditory psychophysics due to a change in the phase of an acoustic signal is in progress, but useful results have not yet been obtained in large numbers. The research results into auditory psychophysics due to a change in the phase of acoustic signals are disclosed by E. Zwicker and H. Fastl, [xe2x80x9cPsychoacoustics-Facts and Modelsxe2x80x9d, Springer-Verlag, 2nd Eds, 1999], and B. C. J. Moore, [xe2x80x9cIntroduction to the Psychology of Hearingxe2x80x9d, Academic Press, 4th Eds., 1997]. According to these documents, the cochlea of the internal ear among hearing organs can be modeled as a filter bank. The filter bank includes band pass filters, and the passband of each filter can be estimated when the central frequency of the filter is given. Signal processing within a human ear has been known as multi-channel signal processing preformed in units of each critical band of the filter.
When a phase change in a signal is considered from this standpoint, a local phase change denotes a change in the relative phase relationship between signal components which exist within the same critical band (i.e., within the same channel). A global phase change denotes that the phase relationship between channels varies while the relative phase relationship between signal components within the same critical band is being kept. The human ear is dull to global phase changes and somewhat sensitive to local phase changes, which is not completely theorized but known in relation to auditory psychophysics with respect to phase. This is disclosed by R. D. Patterson, [xe2x80x9cA Pulse Ribbon Model of Monaural Phase Perceptionxe2x80x9d, J. Acoust. Soc. Am., Vol. 82, No. 5, pp. 1560-1586,1987]; and M. R. Schroeder, [xe2x80x9cNew Results Concerning Monaural Phase Sensitivityxe2x80x9d, J.Acoust. Soc. Am, Vol. 31, p.1579, 1959].
Also, phase information processing in a harmonic speech system is disclosed by R. J. MacAulary and T. F. Quatieri, xe2x80x9cSinusoidal Coding in Speech Coding and Synthesisxe2x80x9d, W. B. Kleijn and K. K. Palivwal Eds, Elsevier, pp. 121-173, 1998; J. S. Marques and L. B. Almeida, xe2x80x9cSinusoidal Modeling of Voiced and Unvoiced Speechxe2x80x9d, in Proc. ICASSP, pp. 203-206, 1983; and J. S. Marques, L. B. Almeida, and J. M. Tribolet, xe2x80x9cHarmonic coding at 4.8 kb/sxe2x80x9d, in Proc. ICASSP, pp. 17-20, 1990. According to these documents, a harmonic speech coding system can be used to express the excitation signal of speech using the following Equation 1:                               e          ⁡                      (            n            )                          =                              ∑                          k              =              1                        K                    ⁢                      xe2x80x83                    ⁢                                    A              k                        ⁢                          cos              ⁡                              (                                                      k                    ⁢                                          xe2x80x83                                        ⁢                                          ω                      0                                        ⁢                    n                                    +                                      θ                    k                                                  )                                                                        (        1        )            
wherein xcfx890 denotes a fundamental frequency, Ak denotes the spectral magnitude of harmonics, and xcex8k denotes the phase of harmonics. The excitation signal is used as the input to a filter which has been modeled by the spectral envelope of speech, to thereby finally obtain an acoustic signal. Thus, in a speech coding system, spectrum envelope filter coefficients, the spectral magnitude Ak, the fundamental frequency xcfx890, and the phase of harmonics (xcex8k) are quantized and transmitted, and acoustic signals are synthesized using the received parameters. In present harmonic speech coding systems, the spectrum phase information xcex8k is relatively neglected compared to the spectral magnitude information Ak of a signal, and a method in which a transmission system does not send the phase information of an acoustic signal, but a reception system applies an arbitrary phase using the condition that the phase of an acoustic signal continuously changes, is generally used.
However, an acoustic signal synthesized by the conventional method does not provide a satisfactory quality of sound. Also, when phase information is completely coded to solve this problem, the amount of information increases too much.
An objective of the present invention is to provide an acoustic signal phase information processing device, in which important phase components are discriminated in consideration of human auditory characteristics to selectively code or synthesize the phase components of an acoustic signal.
Another objective of the present invention is to provide an acoustic signal phase information processing method performed by the above device.
To achieve the first objective, there is provided a device for processing the phase information of a digital speech signal which is expressed as a discrete sum of periodic signals having different frequency components, according to an aspect of the present invention. This device includes: a critical bandwidth calculator for calculating the critical bandwidth of each frequency according to the bandwidth characteristics of a human""s auditory filter; a frequency range setting unit for setting the frequency ranges of local phase changes using critical bandwidths corrected by multiplying the critical bandwidths by a predetermined scaling coefficient; and a phase significance discriminator for checking whether frequency components adjacent to each frequency are within the frequency range corresponding to the frequency, and discriminating whether the phase of a signal having the frequency component is significant in terms of auditory characteristics.
Preferably, the device further includes an acoustic signal transformer for transforming an acoustic signal into the discrete sum of periodic signals having different frequency components. Also, it is preferable that the scaling coefficient is smaller than 1. Preferably, the phase significance discriminator obtains an assembly of frequencies having phases that are significant in terms of auditory characteristics.
To achieve the first objective, a device for processing the phase components of an acoustic signal, according to another aspect of the present invention, includes: an acoustic signal transformer for transforming an acoustic signal into             s      ⁡              (        n        )              =                  ∑                  l          =          1                L            ⁢              xe2x80x83            ⁢                        A          l                ⁢                  cos          ⁡                      (                                                            ω                  l                                ⁢                n                            +                              θ                l                                      )                                ,
wherein L is an integer greater than 1, A1, xcfx89l, and xcex8I denote the spectral magnitude, frequency, and phase of an I-th periodic signal, respectively, and xcfx891 less than xcfx892 less than . . .  less than xcfx89L; a critical bandwidth calculator for calculating the critical bandwidth of each frequency according to the bandwidth characteristics of a human""s auditory filter; a frequency range setting unit for obtaining critical bandwidths xcfx89L,UB and xcfx89l,LB corrected by multiplying the critical bandwidths by a predetermined scaling coefficient, and setting a frequency set of a channel satisfying the condition of xcfx89l,LBxe2x89xa6xcfx89xe2x89xa6xcfx89l with the frequency xcfx89l set as an upper bound, to be C(xcfx89l,1), and setting a frequency set of a channel satisfying the condition of xcfx89lxe2x89xa6xcfx89xe2x89xa6I,UB with the frequency xcfx89I set as a lower bound, to be C(xcfx89l,2); and a phase significance discriminator for discriminating whether the conditions of xcfx89Ixe2x88x921∉C(xcfx89l,1) and xcfx89l+1∉C(xcfx89l,2) are satisfied with respect to xcfx89l, and outputting significance data representing that the phase xcex8I of the frequency xcfx89l is not significant in terms of auditory characteristics, if the conditions are satisfied, and otherwise, outputting significance data representing that the phase xcex8I of the frequency xcfx89l is significant in terms of auditory characteristics.
To achieve the second objective, a method of processing the phase components of an acoustic signal, according to an aspect of the present invention includes: (a) expressing an acoustic signal as a discrete sum of periodic signals having different frequency components; (b) calculating the critical bandwidth of each frequency according to the bandwidth characteristics of a human""s auditory filter; (c) obtaining corrected critical bandwidths by multiplying the critical bandwidths by a predetermined scaling coefficient; (d) setting the frequency ranges of local phase changes using the critical bandwidths corrected in step (c); and (e) checking whether frequency components adjacent to each frequency are within the frequency range corresponding to the frequency, and discriminating whether the phase of a signal having the frequency component is significant in terms of auditory characteristics.
To achieve the second objective, a method of processing the phase components of an acoustic signal, according to another aspect of the present invention, includes: (a) expressing an acoustic signal as             s      ⁡              (        n        )              =                  ∑                  l          =          1                L            ⁢              xe2x80x83            ⁢                        A          l                ⁢                  cos          ⁡                      (                                                            ω                  l                                ⁢                n                            +                              θ                l                                      )                                ,
wherein L is an integer greater than 1, AI, xcfx89l, and xcex8I denote the spectral magnitude, frequency, and phase of an I-th periodic signal, respectively, and xcfx89l less than . . .  less than xcfx89L; (b) calculating the critical bandwidth of each frequency according to the bandwidth characteristics of a human""s auditory filter; (c) obtaining critical bandwidths xcfx89l,UB and xcfx89l,LB corrected by multiplying the critical bandwidths by a predetermined scaling coefficient; (d) setting the frequency xcfx89l as an upper bound and setting a frequency set of a channel satisfying the condition of xcfx89l,LBxe2x89xa6xcfx89xe2x89xa6xcfx89l to be C(xcfx89l,1); (e) setting the frequency xcfx89l as a lower bound and setting the frequency assembly of a channel satisfying the condition of xcfx89lxe2x89xa6xcfx89xe2x89xa6xcfx89l,UB, to be C(xcfx89I,2); and (exe2x88x921) determining the phase xcex81 of the frequency xcfx89l as a phase which is not significant in terms of auditory characteristics, if the conditions are satisfied in step (e); and (exe2x88x922) determining the phase xcfx89l the frequency xcfx89I as a phase which is significant in terms of auditory characteristics, if the conditions are not satisfied in step (e); (f) determining whether I is L, and concluding the process if the I is L, and otherwise, increasing the I by one and returning to the step (e).